Image projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
Color image projection systems may operate on the principle that color images are produced from the three primary light colors: red (“R”), green (“G”), and blue (“B”). With reference to FIG. 1, a prior art image projection system 100 includes a primary light source 102 positioned at the focus of an ellipsoidal light reflector 104 to produce light rays 105 (not shown) of polychromatic light that propagate along a primary light path 106 through a rotating color wheel assembly 108. Color wheel assembly 108 includes at least three filter sections, each tinted in a different one of primary colors R, G, and B. Light rays 105 of polychromatic light emitted by primary light source 102 propagate along light path 106 through an optical integrating device, such as a light tunnel 110 of either a solid or hollow type, to create at its exit end a uniform illumination pattern. (A light tunnel 110 of a solid type is shown in FIG. 1.) Light tunnel 110 works on the principle of multiple reflection to achieve uniform light intensity over a rectangular area with the same dimensional proportions as the final projected image. The illumination pattern is imaged by a lens element system 112, reflected off a light reflecting surface 114, and transmitted through a projection lens 116 to form an image. Popular commercially available image projection systems of a type described above include the LP300 series manufactured by InFocus Corporation, of Wilsonville, Oreg., the assignee of this application.
There has been significant effort devoted to developing image projection systems that produce bright, high-quality color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
To improve the brightness of images they project, image projection systems typically employ a high-intensity discharge (“HID”) arc lamp as primary light source 102. FIG. 2 shows an exemplary HID arc lamp 120 that includes first and second electrodes 122 and 124 separated by an arc gap 126, which is preferably between 0.8 and 2.0 mm wide. First and second electrodes 122 and 124 and arc gap 126 are contained within a sealed pressurized chamber 128 that is filled with ionizable gases and solids. A high voltage pulse applied to first electrode 122 by an external voltage source (not shown) causes ionization of the gases and solids contained within chamber 128 such that a steady state reversible reaction occurs, resulting in the formation of plasma. The current flow developed across arc gap 126 is maintained by external lamp driving electronic circuitry, thereby maintaining the plasma generated by the steady state reversible reaction. The plasma emits bright polychromatic light. The components of arc lamp 120 are enshrouded in a glass envelope 130, and conductive foil plates 132 are attached to electrodes 122 and 124 to dissipate heat and thereby prevent cracking of glass envelope 130.
Thus HID arc lamps produce a point source of intense polychromatic light. Placing the HID arc lamp adjacent to an ellipsoidal reflector allows focusing of the intense polychromatic light with high precision onto a color wheel. HID arc lamps have many favorable attributes, such as high intensity, efficiency, and reliability; but, unfortunately, HID arc lamps typically take some time to warm up, after power on, before achieving their full brightness. During this initial (post power on) period, the brightness of the projected images gradually increases. Resultantly, users of projection systems having such HID arc lamps often feel the projection systems are not fully operational during this warm up period.
Further, the polychromatic light emitted by HID arc lamps is not balanced in terms of its emission energy content. Specifically, HID arc lamps provide greater emission energy content at the blue end of the color spectrum than at the red end, causing an emission energy imbalance. There have been several attempted approaches to solving this problem.
One attempt to minimize illumination emission energy imbalance entailed increasing the angular extent (physical size) of the color wheel R filter segment relative to the angular extent of the B filter segment and/or increasing the attenuation of the color wheel B filter segment relative to the attenuation of the R filter segment. A second attempt entailed reducing overall brightness levels through color lookup electronics to achieve “headroom” for color adjustments. Unfortunately, these attempts either caused temporal artifacts or decreased image brightness. A third attempt entailed adding a white filter segment to the color wheel to provide a “white peaking” function. The addition of a white filter segment increased image brightness but resulted in a loss of brightness of saturated colors. Unfortunately, these optical components caused a significant amount of light to escape from the primary colors. A fourth attempt entailed simply employing a more powerful arc lamp in the projection system. When implemented in compact portable projectors, this method led to heat, size, weight, cost, and reliability issues.
What is needed, therefore, is an image projection system that is implemented with an improved technique for providing a user with an instant-on experience, and/or for achieving increased image brightness and adjustable color balance while reducing light loss during operation.